MOSFET

To continue and correcting Kijanki, especially nowdays, bipolar transistors are substantially higher quality than a couple of decades ago. Hence most of advantages/disadvantages of bipolar transistors vs. MOSFETs disappear (especially for audio applications) such as
- higher bandwidth(no need to go onto MHz ranges I believe)
- more tolerant of difficult loads (never had been more tolerant than bipolar transistors that have substantially lower output impedance with common emitter connection)

Therefore the preference to MOSFET based power amplifier could only be for older vintage models.

They are tools, nothing more or less, some of my favorite amps have them, some do not. The GamuT amps , which I have been considering, have only one pair of transistors a channel, I don't know of any bipolars that can do this for 200 watts a channel. So circuit simplification remains an advantage for them.

"no need to go onto MHz ranges I believe"

In general, to keep things stable output stage should be as fast as possible (little phase shift)and input stage as slow as possible (large phase margin). Power output transistors of modern amplifiers have hundreds of MHz bandwidth (even though slower devices are available). Also smalls signal bandwidth is different (much higher) from large signal bandwidth. We need to keep total amp's bandwidth as high as possible to avoid phase shift in the pass-band. Jeff Rowland uses 180MHz opamps (OPA1632) in Capri preamp. I have impression, so far, that he knows what he is doing.

So that's what it means?! I never knew...and I used to own one, two actually, my old Moscode 300's...I loved em way back when. I used to laugh at the reviewers back then who always went on about the mosfet mist...

09-01-09: Kijanki
There is no perfect device. Mosfets output stages are:

- more robust than transistors
- have higher bandwidth
- are more tolerant of difficult loads

but are:

- less efficient since have lower voltage swing from the same supply voltage
- are more nonlinear than transistors around cutoff region
- require more gain in preceding stages to eliminate crossover distortions
- limited ability to apply local feedback (low gain)

Kijanki, some of the material you have written doesn't make sense to me & also appears to be contradictory: in the advantages you wrote that MOSFET have higher bandwidth than BJTs. In the disadv you wrote that they have lower voltage swing from the same supply voltage. This is contradictory to me.
Lower voltage swing from the same voltage supply compared to the BJT implies lower gain. Bandwidth & gain (Gm) are directly related. Lower gain (for the same amount of bias current) compared to the BJT implies lower bandwidth. This seems to be what I observe too.

I also don't know if there is any substantial evidence of BJTs being less robust than MOSFETs. Ditto re. the ability of MOSFETs to handle difficult loads any better than BJTs.

MOSFETs are a square law device hence the distortions are more along the tube-like behaviour. That's why one often reads of MOSFETs being tubey sounding. In comparison the BJT is an exponential device & the distortions are odd-order harmonic based. If the implementation is not correct/good enough this can lead to very discordant sound very quickly.

I agree w/ most of the other members that MOSFETs should sound pretty good if implemented "correctly" (another audio fuzzy term!). I believe that even better than the MOSFET is its cousin the JFET in the output stage - MOSFET electrical characteristics but BJT-like sound. Preferred by the discriminating amp builders. The sad part is that no one is making large quantities of power JFETs like they are power MOSFETs & BJTs. Thus amps seldom use JFETs.
Now, First Watt *seems* to be using JFETs since they output only a few watts.